Polysilicon is very significant in the manufacture of semiconductor devices. It has very significant uses as the gate of transistors, as interconnect, and as resistors. For use as interconnect, the polysilicon contacts various circuit elements which may require the polysilicon to contact a different layer of polysilicon and/or a semiconductor substrate. A contact between the substrate and a polysilicon layer is generally called a buried contact. A contact between two different polysilicon layers and the substrate is referred to as a shared contact. In both cases there is contact between polysilicon and the substrate which is virtually always monocrystalline silicon. The polysilicon to substrate contact is generally achieved by first exposing the portion of the substrate which is the location of the contact, then depositing a layer of polysilicon. The polysilicon is subsequently masked and etched to remove the polysilicon which is not wanted and to leave at least the portion polysilicon which was deposited on the exposed portion of the substrate. The polysilicon is thus in physical contact with the substrate at the contact location and thus forms the desired contact.
The layer of polysilicon which is deposited is almost certain to have other purposes than simply forming contacts to the substrate. For example, in the case static random access memories (SRAMs), each memory cell in the memory array generally has two loads which are generally formed from polysilicon. In such case, the layer of polysilicon must have very predictable electrical characteristics. In the case of polysilicon, one of the physical characteristics which has a significant impact on the electrical characteristics is the average grain size of the deposited layer. Thus, it is very important to be able to have consistent grain size for the polysilicon layer which is used for the resistors in the SRAM. In order to achieve this, the furnace which is used for the polysilicon deposition must be able to provide a specific temperature to all locations in the furnace where polysilicon is deposited. There are furnaces that can do this but there is a substantial amount of time required in order to achieve the desired temperature, within the narrow tolerance required, at all deposition locations in the furnace.
A natural consequence of having exposed monocrystalline silicon is the formation of a film, such as silicon dioxide (oxide), on the silicon. The exposed portion of the substrate for the contact is an example of this. The contact location is exposed by performing an etch. After performance of the etch the wafers, each of which contain many semiconductor devices, are cleaned in acid and washed in de-ionized water. Oxide formation begins immediately after the wafers have been cleaned by the acid. This oxide formation continues even after the wafers are placed in the furnace. Heat tends to increase the rate of formation of this undesired film of oxide. The oxide formation can continue until the polysilicon is deposited. The oxide formation is not necessarily uniform on the wafers or even on the same wafer or on different contact locations on the same substrate. The tendency, though, is to obtain more oxide as the wafers stay in the furnace prior to polysilicon deposition. The oxide increases the resistance of the contact formed by the subsequent polysilicon deposition. The contact resistance, if it is too great, can adversely affect circuit operation to the point that the circuit operates outside of some specified parameter. This has been found to cause yield loss even to the point of having to scrap whole wafers.